296 ATMOSPHERIC ELECTRICITY. 



pith of elder, half a line or more in diameter, and fitted by little rings 

 to a small metallic stem, which was attached to the upper part of a 

 glass bell two or three inches in diameter. The divisions marked on the 

 bell served to ascertain the number of lines and fractions of lines by 

 which the two little pendulums diverged from each other. Four small 

 slips of tin foil, pasted on the inner surface of the bell, and connected 

 with the metallic base of the instrument, served to carry off the elec- 

 tricity with which the internal air became insensibly charged. In 

 order to determine the proportion which subsisted between each 

 divergence of the balls and the strength of the charge of his elec- 

 trometer, De Saussure* took two of these instruments, as like each 

 other as possible^ and after having electrified one of them so as to 

 of exhibit a divergence of six lines, he touched the rod or stem with that 

 the second ; at the instant, the electricity dividing itself equally between 

 the two instruments, the divergence in each of them was four lines. 

 From this it follows that a diminution of one-half in the intensity of 

 the electricity reduced the divergence but one-third. Eestoring the 

 second electrometer to its natural state, and placing it anew in 

 contact with the first, the divergence was 2.8 ; after a third contact, 

 not more than 1.9 lines. He thus succeeded in arranging the elements 

 of a table which changes with different instruments and which 

 every observer ought carefully to construct for himself before he com- 

 mences a regular series of observations. 



To collect electricity De Saussuref made use, in his first experi- 

 ments, of a fine silver wire, from 50 to 60 feet long ; to one end of which 

 was fastened a ball of lead three or four ounces in weight, and its 

 other end was attached to a hook substituted for the ball on the stem of 

 the electrometer, so that it might be detached by the slightest effort. 

 Holding the instrument in his left hand, he cast into the air with his 

 right the leaden ball, which by this movement carried up the conduct- 

 ing wire and transmitted to the electrometer the electricity which it 

 acquired at its greatest height. But by the continuation of the same 

 movement, the wire drew on the hook, loosened it, and left the elec- 

 trometer charged with the electricity which it had acquired. De 

 Saussure foresaw an objection which might be made to this method of 

 observing atmospheric electricity. " Might not the electricity," said 

 he, " which the ball exhibits be produced by its friction with the air, 

 even though the air may not be charged with electricity ? To decide 

 this question by experiment, I attached the same ball to a cord of 

 silk, and I caused it to turn very quickly in the air ; but it did 

 not by this contract any electricity, which proves that the friction 

 with the air is not capable of electrifying it, and that thus the elec- 

 tricity which is obtained by my method really belongs to the air into- 

 which the ball is thrown." 



This method has also been used by Messrs. Becquerels and Breschet, J 

 to approximate to an estimate of the intensity of the electricity of the 

 atmosphere at different heights above the ground, except they used an 

 arrow instead of a ball. Having ascended one of the plateaus of the 



'-' Voyage dans les Alpes, torn. II, sec. 793, p. 206. 



t Voyage daas les Alpes, torn. II, sec. 785, page 197. 



J Traits d'Electricite et du Magnetisme, par Becquerel, Tom. IV, page 110, Tarls, 183&. 



